Snap, crackle and pop!

[CVSensei]

Can anyone explain what it is that makes an exhaust pop? I believe the technical term is a misfire?

Is it an unhealthy sign or harmless?
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2014 Yamaha MT-07 ABS Deep Armour
'If all else fails, read the instructions.' - Grandpa
'Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.' - Arthur C. Clarke

[Polarbear]

It used to be a sign of an air leak in the exhaust system allowing O2 in to mix with unburnt fuel causing it to ignite.

In modern bikes it seems to be very common but it's a lot less 'vicious' than the backfire I am talking about. Maybe a trait of the modern short exhaust pipe?

The difference between a backfire and a 'burble' on over run is pretty obvious when you hear it.
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Triumph Trophy Launch Edition

[Rogerborg]

I've been wondering this and am interested in a non-Teffers EXPLANATIONE of exactly what's going on.

Enfield, free flow exhaust, no cat, Amal carb, the fuelling seems about right at idle and WOT. No leaks that I can determine.

If I rev it hard (relatively speaking) and then shut the throttle off, after a few seconds I get a few hefty pops and bangs, especially if I down shift.

Science says?
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Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[CVSensei]

[stinkwheel]

Often a sign of running lean. Especially what happens on the overrun when the throttle is closed.

Insufficient fuel charge to ignite properly so it doesn't explode during the power stroke. The fuel:air mix is expelled into the exhaust. Next lean fuel charge comes along, same thing. Perhaps a third.

Eventually one of them ignites and the slightly burning gas exiting into the exhaust ignites the two or three fuel charges that are now filling the exhaust.

Some dirt bikes which are commonly ridden downhill on a closed throttle for protracted poeriods have a "coaster enricher valve" fitted in the carb that is sucked open by the excess vacuum against the closed throttle and allows extra fuel to enter the engine.

With an exhaust air leak, I think it's a bit of the above, less back pressure will tend to exacerbate the whole 8-stroking effect, so people who fit a loud pipe and don't alter the jetting get a drop in power and lots of popping on the overrun. Also a bit of stoichiometry in that any small quantities of unburned fuel will reach explosive air ratios at normal air pressure further up the exhaust system where it is hotter and more likely to ignite them

Of course, on an Enfield, it could also be that your exhaust valve seat has fallen out of the cylinder head and is now flapping up and down on the end of the valve. That causes a lot of popping on a closed throttle due to fuel air escaping into the exhaust on the compression stroke.
https://i48.photobucket.com/albums/f216/stinkwheel/enfield%20head%20231010/dscn1428.jpg
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[Rogerborg]

[c_dug]

My understanding was that by shutting the throttle you close off the air into the cylinder, but some fuel has already been drawn through. This doesn't fully combust because for a cycle or two it throws out the air:fuel ratio. You end up with excess fuel coming out of the exhaust, it is already very hot because of the engine, with the addition of fresh air which it meets exiting the exhaust it ignights, causing the popopopop we all adore so much!
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I am a bellend, I am a man of constant sorrow, I am a gummy bear, I am a rock.

[CVSensei]

Cheers, all of that makes lots of sense.

To conclude, braaaaaaaaaaaaap pap papa pop is fine and dandy?
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2014 Yamaha MT-07 ABS Deep Armour
'If all else fails, read the instructions.' - Grandpa
'Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.' - Arthur C. Clarke

[Seb]

Probably my favourite thing about my triumph and it's underseat exhaust
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2010 Triumph 1050 Sprint ST

[CVSensei]

[Lord Percy]

Yeah as said, it's just fuel (or vapours) that makes it through the carb without being ignited. It then sits in the exhaust until it explodes in there instead.

[deadwolf]

One part of my daily commute is a short, narrow and twisty road looping through a small valley. Its usually filled with slow-moving cars on the ride home which gives plenty of opportunities to kick down a gear and let the engine braking and some mild throttle get the revs up for a nice warm aural sensation before releasing the throttle for some delicious exhaust burbles. There's one long downhill stretch on the final approach that is particularly good for this. Definitely one of the highlights of my commute
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Motorsapien Art

[temeluchus]

On an fuel injected bike, when you close the throttle the injection stops spraying in fuel whilst you are on the overrun. This creates a lean condition that results in the pops if you have a noisier aftermarket pipe. Stock pipe usually shuts this up. Also in stock condition more recently machines run a bit lean, making this even more obvious.

Perfectly normal.
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[Taught2BCauti...]

A Missfire is when the compressed fuel/air charge inside the cylinder doesn't ignite for some reason (usually loss of spark), and passes into the exhaust system unburnt.

Pre-Ignition happens when the fuel/air charge inside the cylinder is ignited by something other than the spark plug at the correct time (usually a glowing ember of carbon, or the heat of compression) also known as 'Pinking', 'Dieseling' or 'Knocking'.

A Backfire is when a fuel/air charge is ignited outside the cylinder, which can be on the inlet side (usually caused by a leaking inlet valve or incorrect valve timing) or on the exhaust side (usually caused by a glowing ember of carbon, or the glowing filament in an O2 sensor).

Negative pressure waves inside the exhaust pipes can sometimes cause oxygen to be drawn back into the system from outside, causing a small amount of super-heated unburnt fuel to spontaineously ignite.

Unburnt fuel particles normally pass through the entire exhaust system without igniting, because they are surrounded by carbon monoxide, carbon dioxide and other gasses that do not support combustion, but a small amount of oxygen introduced through a leak in the system, or by being drawn back into the system by negative pressure, is enough to cause the unburnt fuel particles to ignite.

If the unburnt fuel reaches the outside air, the ratio between oxygen and fuel, and the lower pressure, is not enough to cause ignition.
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[Pol Anorl]

my fazer pops a lot when i downshift off the throttle, sounds badass!
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[Acemastr]

Happens on new bikes because there are valves in the airbox that open to let fresh air into the exhaust to burn off unburnt fuel when you let the throttle off for emissions.

Mine does it... sounds awesome
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2017 KTM 350 XC-F - 2014 Yamaha R6 - 2005 Yamaha YZ125
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[Vincent]

Interestingly, I was reading something online about "backfires" the other day and, as above, what 99% of people know as a backfire, (unburnt fuel igniting in the exhaust) is not so. A backfire is actualy fuel burning in....or out of... the carb. Or so it goes.......
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[Taught2BCauti...]

[Hefty]

Everytime I roll off I get one or two pops, love it. I was actually wondering too if there was anything to be concerned about.
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'13 Honda CBF125 // '99 Suzuki SV650s // '98 Honda CB600F Hornet

[stevo as b4]

I'm not sure that the lower temp/pressure of unburnt fuel mixture reaching the end of the exhaust (atmosphere) is not sufficient for combustion in all cases though.

My car is one of many that sometimes shoots a small flame from the exhaust on the overrun, I think many of the old school non direct injection turbo car's can do this with free flowing exhausts and when the cat is removed. Mine seems to run around 10:1 AFR on full boost, so there's always a lot of fuel left in the exhaust system ready for a source of oxygen/ignition.

[Vincent]

Serious Back Fire:-

https://i77.photobucket.com/albums/j71/Drewskovitch/Jap%2070s%20bike%20toys/Bike%20toys/Fireman_zpsqse2nwib.jpg
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Space Is Deep

[Vincent]

[Polarbear]

It's just terminology though. I bet more people use the term backfire about the bang from an exhaust than blowing the carbs up.

See https://www.youtube.com/watch?v=XVDXLW3P7q8
and https://www.youtube.com/watch?v=bgS3PA5r0vY
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Triumph Trophy Launch Edition

[Vincent]

[Rogerborg]

So what's the terse term for "partially combusted fuel detonating in the exhaust"?
____________________
Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike